A key contemporary engineering challenge is the societal need to improve the conversion efficiency of current steam power plants to reduce carbon gas emission into the environment. The demand for higher thermal efficiency translates into the need to increase the operating temperature of the power plants. This is especially critical for high-temperature alloys such as Cr—Mo steels that are normally used in steam power generation plants. The Cr—Mo steels typically develop Cr-based oxides that are not fully protective at temperatures above 550° C. Oxidation products yield scales that tend to spall so that there is a consequent metal cross section loss, blockage and erosion of components located downstream and overheating.
Higher temperature requirements necessitate structural steels that can sustain the higher temperatures from both a mechanical and environmental perspective. There has been an array of alloying work concentrated on improving the mechanical properties of the candidate high-temperature structural steels, especially the 9Cr-1Mo-0.1C (wt. %) P92 steels which have emerged as a model alloy steel. See, for example, Sawada K, Kubo K, Abe F., “Creep Behavior and Stability of MX Precipitates Reactions During Creep of an advanced 9% Chromium Steel”, Mat. Sci. & Eng. A 2001; 319-321:787-787; Maile K., Klenk A., Roos E., Husemann R-U, Helmrich A., “Development and Qualification of New Boiler and Piping Materials for High Efficiency USC Plants”, Proc. 4th. Int. Conf Advances in Mat, Tech. for Fossil Power Plants, 2005: 152-164; F, Taneika M., Sawada L., “Alloy Design of Creep-Resistant 9Cr Steel Using a Dispersion of Nano-Sized Carbonitrides”, Int. J. Press. Vessels. Pip. 2007; 84(1-2):3-12.; Ennis P. J., Zielinskalipiec A., Wachter O., Czyrska-Filemonowicz A., “Microstructural Stability and Creep Rupture Strength of the Martensitic Steel P92 For Advanced Power Plant”, Acta Materialia, 1997; 45:4901-4907; Brozda J., Pasternak J., “Weldability Evaluation of Martensitic Heat Resisting Chromium Steels with Tungsten Additions and Properties of Welded Joints”, Proc. 4th. Int. Conf. Advances in Mat. Tech. for Fossil Power Plants, 2005: 967-986; and Dryepondt S., Zhang Y., Pint B. A., “Creep and Corrosion Testing of Aluminide Coatings on Ferritic-martensitic Substrates”, Surface & Coatings Technology, 201 (7):3880-3884. The extensive chemical modifications were aimed at enhancing the creep resistance up to 700° C. This involved the addition of elements such as B, Si, V, Nb and W. See, for example, Xiang, Z. D., Datta, P. K., “Relationship Between Pack Chemistry and Aluminide Coating Formation for Low-Temperature Aluminisation of Alloy Steels”, Acta Materialia, 2006; 54:4453-4463; Dryepondt S., Zhang, Y., Pint, B. A., “Creep and Corrosion Testing of Aluminide Coatings on Ferritic-martensitic Substrates”, Surface & Coatings Technology, 201(7):3880-3884; Maziasz, P. J., Shingledecker, J. P., Pint, B. A., Evans, N. D., Yamamoto, Y., More, K., Lara-Curzio, E., “Overview of Creep Strength and Oxidation of Heat-Resistant Alloy Sheets and Foils for Compact Heat Exchangers”, Trans. ASME. The Journal of Turbomachinery, 2006, 128(4):814-819. The modified Cr—Mo steels however remain susceptible to rapid oxidation both in air and, more importantly, in supercritical steam at 650° C.
An in-situ Al-rich iron-aluminide coating has emerged as a leading candidate for high temperature oxidation protection. Unlike chromia or silica, alumina provides excellent oxidation protection even under supercritical steam environments. In this regard, the pack cementation aluminide coating process represents one of the most cost effective and robust methods to coat the alloy steel. The primary obstacle for widespread application of the iron-aluminide coatings has been difficulty in enriching the P92 steel at a low enough temperature, as to avoid degradation of the mechanical properties. The second major obstacle for the use of aluminide coatings has been the concern that the high and prolonged thermal exposure in operation will lead to a severe degradation of the coatings, primarily by means of the depletion of the Al-rich phases within the aluminide coatings through inward diffusion into the substrate.
Until recently, the high temperature requirement for the pack cementation process remained a major obstacle to enrich P92 steels with Al without significant mechanical property degradation. Extensive studies on Al pack cementation using various types of activator (AlCl3, AlF3, NH4F, NH4Cl) pointed to the need for temperatures above 900° C. for the pack process to generate sufficiently high partial pressures of the active carrier gases for the chemical deposition. See, Hocking, M. G., Vasantasree, V., Sidky, P. S., “Coatings by Pack, Slurry, Sol-Gel, Hot-Dip, Electrochemical and Chemical Methods”, Bath Press, Avon, UK: Longman Scientific & Technical, 1989. The conclusion had been that while the aluminide coating can perform very well under a steam environment and provides excellent oxidation protection, the necessary high temperature severely limits its applicability. There have been attempts to examine a low-temperature Al pack aluminizing into P92 steels as a feasible process. See, for example, Xiang, Z. D., Datta, P. K., “Formation of Aluminide Coatings on Low Alloy Steels at 650° C. by Pack Cementation Process”, Mater. Sci. and Tech., 2004, 20:1297-1302; Xiang, Z. D., Datta, P. K., “Relationship Between Pack Chemistry and Aluminide Coating Formation for Low-Temperature Aluminisation of Alloy Steels”, Acta Materialia, 2006, 54:4453-4463. The work performed by Xiang et. al., using AlCl3 activator and an Al depositing powder source, demonstrated the ability to coat Cr—Mo steels. See, for example Xiang, Z. D., Datta, P. K., “Relationship Between Pack Chemistry and Aluminide Coating Formation for Low-Temperature Aluminisation of Alloy Steels”, Acta Materialia, 2006, 54:4453-4463.